The Nervous System - 2

The vertebrate nervous system is divided into two major divisions: the central nervous system (CNS) and the peripheral nervous system.

The CNS has two major parts: the brain, which is the center of neural integration; and the spinal cord, which consists of billions of neurons that connect with the brain and house the apparatus for reflex arcs.

All other neurons outside the CNS are a part of the peripheral nervous system and are referred to as nerves-- living cables of neurons bundled together in a parallel arrangement.

It is important that reaction times be rapid. The fastest of the motor reactions to various stimuli are reflex responses.

A reflex is an involuntary response to a stimulus. This sort of response is automatic and requires no conscious thought on the part of an individual. The stretch reflex that your physician checks during a physical examination by tapping the region just below your kneecap is one such response.

The patellar reflex works like this: A sensory receptor-- a neuron-- has its end wrapped around a muscle fiber embedded within a muscle of the thigh. When the tendon of the knee is stretched, the receptor is activated and generates a neural impulse. Some of the terminal processes of the sensory neuron end in the spinal cord associated with synapses that lead to motor neurons that ennervate thigh musculature. Impulses from these motor neurons cause the muscle to contract and the leg extends forward-- the so-called "knee jerk response."

This reflex did not evolve as a convenient test for physicians. Instead, it functions to help you maintain your posture and balance. You have a large number of reflexes besides the patellar reflex. These include those that control your blood pressure, that cause the pupils of your eye to dilate or contract in response various light conditions, that cause you to move your hand rapidly when it encounters something hot or sharp.

Reflexes are involuntary responses that occur to simple stimuli in a stereotypical manner.

We also have more complex neural circuits. That is, we are interconnected. A single motor neuron may be covered with synaptic knobs from thousands of different neurons. This allows many different motor neurons leading to a musculature to be activated. Information is also shunted upwards to the brain as positional awareness.

The human brain is about 3 pounds of gelatin-like tissue. Its outer region is darker in color and referred to as grey matter. This is where the cell bodies and dendrites of the brain's neurons reside. Grey matter is rich in neuron to neuron synapses.

The more inner portions of the brain are lighter in color and referred to white matter. The white color is due to the myelin-coated axons that are present here. Within the brain, there are chambers or ventricles filled with a fluid-- the cerebrospinal fluid. This stuff acts largely as a cushion.

The brain and its fluids are surrounded by a water-tight sheath called the meninges. Inflammation of this sheath material is called meningitis, a life-threatening situation.

Anatomists divide the brain up into various parts. One common scheme is refer to three functional groups: the cerebrum, the cerebellum and a system of interrelated components consisting of the brain stem, the limbic system and the reticular formation.

The cerebrum is the most prominent portion of the brain. It is split into two halves or cerebral hemispheres which are generally associated with so-called "higher" brain functions-- things like speech and rational thought.

In fact, the higher brain functions are more specifically associated with the cerebral cortex, the wrinkled outer layer of the cerebrum. Consider that every cubic inch of this layer contains perhaps 10,000 miles of interconnecting neurons.

Why the wrinkledness? Well, it increases the surface area and allows more room for more neurons. These convolutions or wrinklings are associated with higher cerebral capabilities. Amphibians and reptiles have smooth brains.

Each of the cerebral hemispheres has 4 lobes: frontal, temporal, occipital and parietal-- each of which has distinct functions.

The cerebral hemispheres are connected by a thick cable of hundreds of millions of neurons-- the corpus callosum.

Forty-three years ago, I was stationed at a Naval Air Station in El Centro, California. In my mind's eye, I can still picture the building I lived in and the faces (though not the names) of some of the people that I worked with at that time.

I can easily remember the appearance of my newborn children and they are all now adults.

How does this work? How is information stored in the cerebral cortex?

Actually, we don't know.

At the same time, we do have some insights. During the 1950s, a young man named HM had had a portion of his brain removed during surgery to stop severe seizures. The operation had unexpected results. HM was able to remember events from his childhood and adolescence but unable to remember any new information. If he was given a number to memorize, he could do so only if he focused all of his attention on it and would forget it as soon as his attention moved to something else.

The case of HM revealed for the first time, the dual nature of human memory: we have long-term memory and short-term memory. They apparently involve different processes and different parts of the brain.

Short-term memory seems to be processed by the temporaral lobes of the cerebrum and the hippocampus, a part of the brain limbic system. These are the same regions of the brain that degenerate in individuals with Alzheimer's disease.

Learning is a change in behavior that results from previous experiences. We know most about learning from studies on a Aphysia, a sea slug. In Aphysia, learning is associated with structural modifications of synapses. In humans, these changes occur primarily in the cerebrum.

Language is another complex phenomenon involving the nervous system. For most people, especially right handed people, the ability to learn a language is localized in the left frontal lobe of the cerebrum.

Regions specialized for receiving sensory input are localized in the cerebral cortex. Other regions are specialized for starting motor output. The motor cortex found in the frontal lobe of the brain is where most of the impulses that command muscles emanate.

The cerebellum is a bulbous structure that receives information from muscles, joints, the eyes and the ears. Cells of this region receive huge numbers of synaptic connections. People who sustain cerebellar damage often have difficulties performing smooth motor activities and have to think carefully about movements that would otherwise have been very automatic.

The brainstem is the central stalk of the brain and is responsible for regulating involuntary actions such as breathing and swallowing and maintaining heart beat rate and blood pressure. Damage to the brain stem usually leads to coma and/or death.

The brainstem is "old" brain. The cerebral cortex is probably the most recently evolved part of the brain-- the brainstem is probably the oldest and makes up the bulk of the brain of lower vertebrates.

The route for sensory information that streams into the cerebral cortex is through a region just below the cerebrum-- a region called the thalamus. The thalamus also coordinates outgoing signals.

Associated with the thalamus is the reticular formation, a small portion of the brain about the size of your little finger. The reticular formation can be activated by a number of input signals. When the reticular formation becomes inactive, you fall asleep.

The reticular system also acts as a screening tool. It filters out stuff, allowing only "vital" information to pass to the brain.

Just beneath the thalamus is the hypothalamus, a portion of brain only about the size of your thumb. This region is responsible for maintaining homeostasis. Here is the seat of body temperature regulation. The hypothalamus also helps control blood pressure, heart beat rate and urges such as hunger, thirst and sex drive.

Finally, the hypothalamus also controls the pituitary gland, an extremely important component of the endocrine system.

The brainstem merges with the second major component of the CNS, the spinal cord. The spinal cord is a thick-walled tubular mass about the diameter of your little finger. It extends from the top of your neck to the bottom of your back.

It is also surrounded by meninges which in turn, is surrounded by the bony vertebra-- your backbone. One also finds cerebrospinal fluid in the spinal column that communicates with the ventricles and fluid found in the brain.

One finds white matter-- myelinated axons and grey matter-- dendries and cell bodies-- in the spinal cord. Their arrangement is reversed, however. The spinal cord's white matter surrounds the grey matter.

The peripheral nervous system acts as a bridge between the CNS and the various parts of the body. Twelve pairs of cranial nerves extend from the central stalk of the human brain; 31 pairs of spinal nerves extend from the spinal cord outward in the human body.

Cranial nerves mostly innervate the tissues and organs of the head and neck; spinal nerves innervate the chest, abdomen and limbs.

The peripheral nervous system has somatic and autonomic divisions. The former carries messages to the skin and muscles that move the skeleton. These muscular motions are generally voluntary movements.

The latter, the autonomic division involves signals that control the involuntary activities of the body's internal organs such as blood vessel diameter, glandular secretions, digestion, sexual responses, etc.

The autonomic division is further subdivided into sympathetic and parasympathetic divisions. The sympathetic division involves the "fight-or-flight" response. Placed in a stressful situation, your sympathetic nervous system triggers an accelerated heart beat rate, blood gets shunted from your extremities to your lungs for better oxygen exchange, the liver is stimulated to release glycogen for conversion to glucose and subsequent oxidation for its stored energy. You are set up to "fight" or "run."

The parasympathetic division is more of a housekeeper division. Digestion and excretion are examples of its tasks.

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